The Ative guys has a blog with a lot of insightful development project gotchas. This one , in particular, made me chuckle.
Although I've never had to introduce bugs on purpose into a system, I think this is is a very elegant solution to the described...

For most of the time, I write code using the test-driven development methodology. This means that I write a bit of code, run a suite of tests, write a bit more code, run some more tests, etc. For this reason, I need unit testing to be an integrated part...